1. Field of the Invention
The present invention relates to a thermal printer and a control method therefor.
2. Description of the Related Art
A conventional thermal printer of this type is provided with a thermal head unit and a paper feed motor as heat generating bodies. Therefore, in order to prevent these heat generating bodies from being heated and burnt out, the conventional thermal printer and a control method therefor are constituted such that a temperature detecting device is attached to each of the thermal head unit and the paper feed motor, and that outputs from the temperature detecting devices are monitored by a control circuit, and power supply to the thermal head unit and the paper feed motor is inhibited so as to prevent each temperature of the thermal head unit and the paper feed motor from exceeding a prescribed allowable value.
In the following, several prior art examples will be described with reference to the accompanying drawings.
First, the whole structure of a thermal printer disclosed in Japanese Patent Application Laid-Open No. 5-4371 is explained as a first prior art example by using a block diagram shown in FIG. 10.
When the temperature of a thermal head is changed, information indicating the temperature change is converted into a digital signal in a temperature control circuit. Further, environmental temperature information is added to the digital signal in the temperature control circuit, and the resultant signal is sent to an oscillation circuit in a motor control circuit. A signal sent from the oscillation circuit is a reference signal for the rotation of the motor, and hence, the rotation speed of the motor is changed in association with the reference signal. That is, the speed of the motor is changed with the temperature change in the thermal head. Further, when the environmental temperature in the device is changed, the rotation speed of the motor is also changed similarly to the case where the temperature in the thermal head is changed. Further, information from a system controller is also added in the temperature control circuit. As a result, the necessary speed change of the motor can be performed from the system controller, in addition to the amount of speed change of the motor based on the temperature change in the thermal head.
Next, a system structure of a label printer disclosed in Japanese Patent Application Laid-Open No. 8-1985 is explained as a second prior art example by using a block diagram shown in FIG. 11.
The temperature of a power supply section which controls power supplied to a print head is detected by a temperature sensor. An interval time corresponding to the temperature detected by the temperature sensor is read from storing means, and a label is issued at the interval time. Thus, when the temperature of power supply section is raised, the interval time for issuing the label is increased, thereby preventing the thermal destruction of the power supply section.
Next, a line thermal printer disclosed in Japanese Patent Application Laid-Open No. 11-179991 is explained as a third prior art example by using a block diagram shown in FIG. 12.
When the temperature of a paper feed motor is raised by continuous printing, a step-out speed region is expanded by a decrease in the motor feed torque. The expansion of the step-out speed region is at a maximum when the temperature of the paper feed motor reaches a maximum temperature value. In the case where the step-out speed region is not controlled in response to the temperature rise of the paper feed motor, the step-out speed region is set to one at the maximum temperature value of the paper feed motor. As a result, when the temperature of the paper feed motor is low, the step-out speed region is set more widely than needed. Accordingly, the motor speed in deceleration is subjected to deceleration correction, so that the printing speed is lowered. On the other hand, by switching the step-out speed regions with the temperature of the paper feed motor, it is possible to prevent the deceleration correction from being performed more than needed and suppress the printing speed reduction.
Next, a thermal printer disclosed in Japanese Patent Application Laid-Open No. 2001-191577 is explained as a fourth prior art example by using a block diagram shown in FIG. 13.
When the temperature of a thermal head is raised, an error may occur in a CPU so that the CPU fails to function normally. In such a case, the temperature of the thermal head is further raised to cause a voltage V to be further decreased. Finally, when the voltage V drops below a reference voltage, the output of an operational amplifier CMP reaches the L level (LHERR=L). This L level signal is a head error signal. As a result, the output of a two-input AND gate G reaches the L level so that TR1 and TR2 are turned off. This interrupts supply of head drive current from a head drive power supply Vp to the thermal head 4. In this way, even when the CPU fails to normally function, it is possible to recover a safe state from the heated state of the thermal head.
Next, a state of a paper feed motor and a thermal head in a thermal printer based on the knowledge of the present inventors is explained as a fifth prior art example by using FIG. 14.
When a print command is received at a control circuit (not shown), a paper feed motor 105 is driven by control of the control circuit, and a paper sheet (not shown) is conveyed, and a thermal head 103b is driven by control of the control circuit for printing on the paper sheet.
The thermal head 103b and a heat dissipating member 103a constitute a thermal head unit 103. The heat generated by the thermal head 103b is dissipated to the outside air through the heat dissipating member 103a to some extent. However, when such printing operation is continued, the temperature of the paper feed motor 105 and the thermal head 103b is successively raised. In the case where the environment temperature is high, the temperature may exceed an allowable temperature at a certain point of time, thereby causing the paper feed motor 105 and thermal head 103b to be burnt out. On the other hand, a heat radiating member such as the heat-dissipating member 103a of the thermal head unit 103 is not provided to deal with the heat generated by the paper feed motor 105. This makes the temperature rise in the paper feed motor 105 severer than the temperature rise in the thermal head 103b. 
In order to detect temperatures of the thermal head, the power supply section or the paper feed motor, and to prevent the respective detected temperatures from exceeding an allowable value, in the first prior art example (in FIG. 10), the rotation speed of the motor is controlled on the basis of the temperature of the thermal head and the environmental temperature information, thereby suppressing the temperature of the thermal head from being further raised. In the second prior art example (in FIG. 11), when the temperature of the power supply section supplying power to the thermal head is raised, the interval time at which the label is issued is increased, thereby preventing the power supply section from being thermally destructed. In the third prior art example (in FIG. 12), the step-out speed region is switched in response to the temperature of the paper feed motor, thereby preventing the deceleration correction from being performed more than needed and suppressing a decrease in the printing speed. In the fourth prior art example (in FIG. 13), in the case where the temperature of the thermal head is raised so as to be ready to exceed an allowable value, when an error occurs in the CPU, TR1 and TR2 are made to interrupt the supply of head drive current to the thermal head 4.
In the fifth prior art example (in FIG. 14), in order to prevent the paper feed motor 105 and the thermal head 103 from being burnt out, a motor temperature detecting device 111 is attached to the paper feed motor 105, and a head temperature detecting device 110 is attached to the thermal head 103, so as to prevent the respective temperatures of the paper feed motor 105 and the thermal head 103 from exceeding an allowable temperature. Then, when the temperature of the paper feed motor 105 is ready to exceed the allowable temperature, an output is produced by the motor temperature detecting device 111 toward the control circuit. On the other hand, the control circuit which receives the output inhibits the power supply to the paper feed motor 105. Similarly, when the temperature of the thermal head 103 is ready to exceed the allowable temperature, an output is produced by the head temperature detecting device 110 toward the control circuit, and the control circuit which receives the output inhibits the power supply to the thermal head 103.
As described above, a conventional temperature control device for preventing heat generating bodies, such as a thermal head, a power supply section, or a paper feed motor from being heated and burnt out, is constituted such that a temperature detecting device is attached to each of the heat generating bodies, and that a control circuit monitors the output from the temperature detecting device and thereby inhibits the power supply so as to prevent the respective temperatures of the heat generating bodies from exceeding an allowable value. Therefore, such structure is expensive because a temperature detecting device is attached to each of the heat generating bodies.